Polyetherimides are unique polymers which exhibit superior physical and chemical properties, including high heat resistance, exceptional strength, and excellent processability. These polymers can be used as wire coatings and are particularly suited for injection molding applications.
A number of processes for making polyetherimides have been disclosed. Generally, these polymers are prepared by reacting an organic diamine with a aromatic bis(ether dicarbonyl), i.e., an aromatic bis(ether anhydride) or an aromatic bis(ether dicarboxylic acid). Two processes which have been of particular interest are the so-called melt polymerization and solution polymerization processes. The basic melt polymerization process was described by T. Takekoshi and J. Kochanowski, U.S. Pat. No. 3,803,805. This process involves combining an aromatic bis(ether anhydride) and an organic diamine and heating the mixture under an inert atmosphere to form a homogeneous melt. Water formed during the polymerization reaction is removed at a temperature of up to 350.degree. C. In a preferred embodiment of the process, the final stage of the reaction is conducted under reduced pressure to facilitate removal of water. The basic polyetherimide polymerization technique has been improved by employing catalysts to enhance yields or reaction rates (for example, see Takekoshi, et al. U.S. Pat. No. 3,833,544 and F. Williams III, et al., U.S. Pat. No. 3,998,840, and Takekoshi, U.S. Pat. No. 4,324,882). In addition, the melt polymerization method has been adapted to the continuous mode by conducting the reaction in extrusion apparatus (For example, see Takekoshi, et al U.S. Pat. No. 4,011,198 and Banucci, et al. U.S. Pat. No. 4,073,773.)
Solution polymerization is generally conducted reacting an aromatic bis(ether anhydride)and an organic diamine in an inert solvent at temperatures up to about 200.degree. C. With this procedure, water of reaction is typically removed by azeotropic distillation. The resulting polymer is generally recovered by mixing the reaction solution with a precipitant, such as methanol. The reaction solvents employed for solution polymerization reactions are selected for their solvent properties and their compatibility with the reactants and products. High-boiling nonpolar organic solvents are preferred. (E.g., see Takekoshi, et al., U.S. Pat. No. 3,991,004.) Dipolar, aprotic solvents and phenolic solvents can also be used, particularly when an aromatic bis(ether dicarboxylic acid) is used as the starting material (e.g., see Takekoshi, et al., U.S. Pat. No. 3,905,942).
Although the foregoing procedures have been used effectively to produce polyetherimides of high quality, they do suffer from certain disadvantages. The principal problems associated with the melt polymerization technique involve controlling the stoichiometric ratio of the reactants during the course of the reaction. Economical production of a polymer having the desired physical and chemical characteristics usually requires controlling the relative proportions of anhydride, diamine and any chain termination agent that is employed. Because of the relatively high temperatures employed in the melt polymerization process and the disparate volatalities of these components, controlling the stoichiometry of the mixture has proven difficult. A further disadvantage of conventional melt polymerization techniques is that the reaction mixtures pass through a so-called "cement stage" as polyamide acid intermediate is formed. During this phase of the reaction, the reaction mixtures become very viscous and difficult to process. D. Heath and J. Wirth (U.S. Pat. No. 3,847,867) disclose a method for preparing polyetherimides which involves stirring a solution of an aromatic bis(ether anhydride) and an organic diamine in a dipolar, aprotic solvent under ambient conditions to produce a polyamide acid and casting the polyamide acid solution on a substrate to facilite the removal of the organic solvent. The cast polyamide acid film can then be heated at temperatures of 150.degree. C. or higher. After the initial heating, the cast film can then be heated to temperatures of from 200.degree. C.-300.degree. C. to convert the polyamide acid to the polyetherimide. While this solution process has some advantages over the melt polymerization due to the complexity and cost of melt polymerization, the subsequent treatment at temperatures up to 300.degree. C. is undesirable. Polyetherimides generally have glass transition temperatures ranging from about 170.degree. to 240.degree. C. If one heats a polyamide acid in excess of the glass transition temperature of the final product, the polyamide particles stick together, become partially molten and thus do not provide a uniform imidization of the polyamide acid.
Accordingly, there is a continuing need for an efficient process for producing high quality polyetherimides.